The present invention relates to a process for production of an olefin polymer, more particulary an EPDM tetrapolymer, having long chain branching.
The polymerization of olefins is well known in the art. It is known to produce both crystalline and amorphous polyolefins via the so-called Ziegler-Natta polymerization process.
Generally, the polymerization reaction is catalyzed through the use of a transition metal catalyst compound and a cocatalyst compound. More specifically, it is conventional to produce EPDM (ethylene-propylene-diene-methylene) terpolymers and EPM (ethylene-propylene-methylene) copolymers in solution or slurry processes using Ziegler-Natta catalysts such as VOCl3, V(acac)3 (acacxe2x89xa1acetylacetonate) and VCl4, in combination with an aluminum-based cocatalyst such as diethyl aluminumchloride (DEAC) and/or ethyl aluminum sesquichloride (EASC) and/or ethyl aluminum dichloride (EADC).
Since EPM copolymers typically have low crystallinity, they are highly soluble in saturated hydrocarbon solutions. For this reason, most of the processes used to produce EPDM terpolymers are solution-based. In these processes, as long as the solution viscosity is kept low, very homogenous polymerization conditions can be maintained. At high solution viscosities, mixing becomes difficult and mass transfer limitations occur resulting in the presence of concentration gradients.
Another known process is based on suspension technology in which the EPDM terpolymer is precipitated in situ as discrete particles in a non-reacting diluent. The fluid phase viscosity remains low, enabling good mixing.
The majority of the current EPDM terpolymer production processes employ soluble Ziegler-Natta catalysts for the production of high molecular weight elastomers. These soluble catalysts are typically formed from vanadium compounds in the oxidation state +3 to +5. Examples of such compounds include vanadium trisacetylacetonate (V(acac)3), vanadium tetrachloride (VCl4) and vanadium oxytrichloride (VOCl3). These catalysts are used in conjunction with organoaluminum cocatalyst compound such as triethyl aluminum, DEAC or EASC.
The acidic catalyst system VOCl3 and EASC (G. Ver Strate in Ethylene-Propylene Elastomers, Ency. Poly. Sci. and Eng., 2nd Ed., Vol. 6, p522 (1986)) is the catalyst of choice for most of the EPDM elastomers produced by solution polymerization. Ethylidene norbornene (ENB) is the common diene used in the commercial production of EPDM elastomers. ENB-based EPDM elastomers produced under solution conditions in the presence of the acidic catalyst system VOCl3 and EASC are branched through cationic coupling of the ENB pendent double bond. It has been reported by Ver Strate et al. (Ver Strate, Kresge, and Cozewith, ACS Rubber Division Meeting, Detroit, Mich., May 1 (1973), Paper #7) that the above combination of catalyst-cocatalyst (i.e., VOCl3/EASC) produce an EPDM elastomer having unimodal molecular weight distribution. However, the use of a less acidic cocatalyst, such as DEAC, resulted in production of a polymer which exhibits multimodal molecular weight distributionxe2x80x94this is disadvantageous.
U.S. Pat. No. 5,674,613 [Narayanaswami et al.], International publication number WO 97/00286 [Ravishankar et al.], International publication number WO 97/00288 [Ellul et al.] and International publication number WO 97/00289 [Jourdain et al.] teach substitution of ENB in a conventional EPDM elastomer with 5-vinylidene-2-norbornene (VNB). According to these references, the resulting elastomer is characterized by improvements in extrusion properties, electrical properties and cure properties compared to EPDM elastomers containing a diene monomer other than VNB. The acidic catalysts systems (e.g., vanadium oxytrichloride/vanadium tetrachloride in combination with DEAC, EASC, etc.) taught in these references for production of the EPDM elastomers are those conventionally used in solution polymerization of EPDM elastomers. The resultant elastomers have a very high molecular weight distribution (MWD)xe2x80x94i.e., most preferably above 15.
It is also known to conduct suspension polymerization (also referred to as slurry polymerization) using a catalyst system consisting of VOCl3 as catalyst and EASC as cocatalyst. Further, it is known to conduct suspension polymerization processes by employing a catalyst system consisting of V(acac)3 and DEAC. It is further known that the VOCl3/EASC catalyst/cocatalyst system is more acidic than the V(acac)3/DEAC catalyst/cocatalyst system. It has also been described that the degree of long chain branching is affected by the acidity of the cocatalyst in the VOCl3 catalyst system. That is, an increase in the cocatalyst acidity increases long chain branching in the EPDM terpolymerxe2x80x94see E. N Kresge, C. Cozewith and G. Ver Strate, ACS Rubber Division Meeting, Indiana, May 8, 1984; and E. K. Easterbrook and E. K. Kontos, Polymer and Fiber Science, VCH Publishers, N.Y., Chapter 27 (1992). In suspension polymerization where V(acac)3 is the catalyst and DEAC the cocatalyst, the degree of long chain branching of the EPDM polymer is low.
It is known that the presence of long chain branching in EPDM polymers at various levels improves their cold flow and processing characteristics (E. N. Kresge, C. Cozewith and G. Ver Strate, ACS Rubber Division Meeting, Indiana, May 8, 1984; and K. P. Beardsley and R. W. Tomlinson, ACS Rubber Division Meeting, Detroit, Mich., Oct. 17, 1989). Thus, control of the degree of long chain branching is desirable for tailoring the properties of the resultant polymer to specific applications.
In published European patent application 0,751,155A (Enichem), a process for preparing ethylene-propylene copolymers in suspension is disclosed. The catalyst system employed requires a catalyst containing vanadium in +3 or +5 oxidation state. The vanadium catalyst is premixed in a hydrocarbon solvent with a cocatalyst having the formula RnAlXm, wherein R is C1-C20 alkyl radical, X is halogen, m+n is 3 and n is an integer from 0 to 2. This reference recognizes DEAC as a cocatalyst.
Notwithstanding the foregoing advances in the prior art, there is an ongoing need to have a practical means of introducing long chain branching to an EPDM terpolymer. It would be particularly advantageous if this could be achieved with an otherwise conventional catalyst system. It would be further particularly advantageous if this could be achieved without substantially broadening the molecular weight distribution (MWD) of the polymer product.
It is an object of the present invention to provide a novel process for production of an olefin polymer which obviates or mitigates at least one of the above-identified disadvantages of the prior art.
Accordingly, in one of its aspects, the present invention provides a process for production of an ethylene-propylene-diene-methylene (EPDM) tetrapolymer having long chain branching, the process comprising the step of polymerizing a monomer mixture comprising ethylene, propylene, a first diolefin monomer containing one polymerizable double bond and a second diolefin containing two polymerizable double bonds in the presence of a catalyst system comprising:
a catalyst comprising a compound containing vanadium +3 with the proviso that the compound does not comprise a halogen directly bound to the vanadium;
a halogenated organoaluminum cocatalyst having a halogen to aluminum molar ratio in the range of from about 1 to about 2; and
an activator.
It has now been found that useful EPDM tetrapolymers, with enhanced long chain branching, can be obtained by the combination of: (i) polymerizing a monomer mixture comprising ethylene, propylene, a first diolefin monomer containing one polymerizable double bond (preferably 5-ethylidene-2-norbornene) and a second diolefin containing two polymerizable double bonds (preferably 5-vinylidene-2-norbornene), and (ii) conducting the polymerization in the presence of a catalyst system comprising a catalyst compound containing vanadium which does not have a halogen directly bound to the vanadium, preferably a catalyst compound containing vanadium (xcex2-diketonate) in an oxidation state of +3, more preferably vanadium acetylacetonate (V(acac)3) or tris(2-acetylcyclohexanone) vanadium, and a halogenated organoaluminum cocatalyst, such as diethylaluminum chloride (DEAC) and/or ethylaluminum sesquichloride (EASC) wherein the halogen (preferably chlorine) to aluminum molar ratio is in the range of from about 1 to about 2.